1. Go into multi-player match and choose offline 2 player (split-screen non-lan).
2. Switch unlimited weapons to ON.
3. Press X to accept the game criteria.
4. Back out and go to One Player Story Mode
5. Highlight a game file that you have completed at least the first mission on. This file cannot be 100 or 0 percent complete.
6. Press Start to RESUME (Not X)

This will need to be repeated each time you want the unlimited weapons ONLY if you leave one player story mode for any reason.
IF YOU JUST PLAY BACK TO BACK MISSIONS WITHOUT LEAVING THE MISSION SELECT SCREEN THE GLITCH WILL NOT RESET.

This glitch accomplishes a number of things.
1. Provides you with unlimited weapons and mega-gun bullets.
2. Energy, turbo, and health are NOT unlimited.
3. Removes ALL pickups from the map EXCLUDING health and turbo pickups.

THE DOWNSIDE:
This may need further testing, but I have reason to believe that you are not the only one with unlimited weapons.
All bots have unlimited weapons as well.
When I started a match, I noticed that two bots already had mega-gun bullets and were shooting me with them.
Now they don't missile spam on Normal difficulty, have not tried the higher difficulties yet.